Liu Lori G, Clark Christopher J
Department of Evolution, Ecology, and Organismal Biology, University of California, Riverside, Riverside, CA 92521, USA.
J Exp Biol. 2025 Jan 15;228(2). doi: 10.1242/jeb.246234. Epub 2025 Jan 27.
One feather structure associated with an owl's ability to fly quietly is the soft dorsal surface on their flight feathers: the velvet. This velvet is a mat of elongated filamentous pennulums that extend up from feather barbules. The aerodynamic noise hypothesis posits this velvet reduces aerodynamic noise caused by the formation of turbulence, while the structural noise hypothesis posits the velvet acts as a dry lubricant, reducing frictional noise produced by feathers sliding past one another. We investigated the structural noise hypothesis by quantifying the length of the velvet on 24 locations across the wing of the barred owl (Strix varia) and then qualitatively assessing the presence of velvet in 24 bird species. We found that velvet has evolved at least 4 times independently (convergently) in owls, nightbirds, hawks and falcons. Then, we rubbed 96 pairs of feathers together from 17 bird species (including the four clades that have independently evolved velvet) under three experimental treatments: control, hairspray applied (to impair the velvet) and hairspray removed. The sound of feathers rubbing against each other was broadband, similar to the sound of rubbing sandpaper or Velcro. Species with velvet produced rubbing sounds that were 20.9 dB quieter than species without velvet, and velvet-coated feathers became 7.4 dB louder when manipulated with hairspray, while feathers lacking velvet only increased in loudness by 1.7 dB, relative to the control treatments. These results all support the hypothesis that the velvet primarily functions to ameliorate the sounds of feathers rubbing against other feathers.
绒羽。这种绒羽是一层从羽小枝向上延伸的细长丝状羽支的垫子。空气动力学噪声假说认为，这种绒羽可减少由湍流形成引起的空气动力学噪声，而结构噪声假说则认为绒羽起到干润滑剂的作用，减少羽毛相互滑动产生的摩擦噪声。我们通过量化横斑林鸮（Strix varia）翅膀上24个位置的绒羽长度，然后定性评估24种鸟类中绒羽的存在情况，来研究结构噪声假说。我们发现，绒羽在猫头鹰、夜鸟、鹰和隼中至少独立进化了4次（趋同进化）。然后，我们在三种实验处理下，将17种鸟类（包括已独立进化出绒羽的四个分支）的96对羽毛相互摩擦：对照、喷洒发胶（以破坏绒羽）和去除发胶。羽毛相互摩擦的声音是宽带的，类似于摩擦砂纸或魔术贴的声音。有绒羽的物种产生的摩擦声比没有绒羽的物种安静20.9分贝，用发胶处理后，有绒羽覆盖的羽毛声音增大7.4分贝，而没有绒羽的羽毛相对于对照处理仅增大1.7分贝。这些结果都支持了绒羽主要功能是减轻羽毛相互摩擦声音的假说。
